The present invention relates generally to digital motor speed regulator systems providing a method and apparatus for controlling the flow of power from an AC source to a load such as a DC motor. More specifically, the present invention relates to a digital control system for the direct digital firing of controllable rectifiers placed between an AC source and a DC motor for controlling the conduction of such rectifiers.
Motor control systems of the type described herein frequently use power amplifiers having controllable rectifiers which vary the flow of electrical energy between an alternating current source and a drive motor. Controllable rectifiers are well known in the art and consist generally of a family of devices which present a relatively high impedance to the flow of electrical energy until such time as they are forward biased and have a firing signal applied to a gate electrode. At the time of conduction, the controllable rectifier provides a very low impedance to the flow of current and will normally continue to conduct current until it is back biased and/or the level of the current flowing through the rectifier is decreased below a minimum holding level necessary to keep the rectifier conducting. The family of controllable rectifiers being discussed generally includes semiconductor devices such as thyristors, the most common type of which is the silicon controlled rectifier (SCR) and other devices such as ignitrons and thyratrons.
In systems of the type being described, the amount of power transferred to a system load or DC motor is controlled by varying the duration of conduction of the controllable rectifiers. Generally speaking, the duration of conduction of the controllable rectifiers is a function of the point during the AC wave form in which they are initiated into conduction. This point is referred to as the firing angle.
In the past, systems for controlling the conduction of controllable rectifiers have generally been implemented using analog control devices to perform the required regulating functions and converting the analog signals into digital values to fire the rectifiers. In these types of systems, the firing circuits act in response to an input signal indicative of desired power to generate a firing pulse at the appropriate firing angle. Generally speaking, the firing angle is proportional to the input signal and analog systems of the prior art generally operate in response to an input signal whose magnitude indicates the desired firing angle.
In recent years however, with the development of digital techniques and hardware, engineers have become interested in utilizing digital circuitry in control systems. The application of digital techniques to such control systems is particularly advantageous where the system requires that degree of accuracy, reliability and/or drift free operation which is available only with digital circuitry. Hence, it is becoming fairly common to replace elements of an analog system with functionally equivalent digital circuitry.
One such known digital control system for controlling the conduction of controllable rectifiers is disclosed in U.S. Pat. No. 3,601,674 entitled "CONTROL SYSTEM FOR FIRING SCR'S IN POWER CONVERSION APPARATUS" to John A. Joslyn and Albert F. Koch, issued Aug. 24, 1971 and assigned to the assignee of the present invention. In that patent, a digital control system is disclosed for controlling the flow of power through controllable rectifiers from a multiphase AC source to a load. That system includes a firing circuit for each phase, wherein each firing circuit comprises a reversible counter and a digital comparator. Phase detection logic is incorporated which examines the three phases of the AC source so as to synchronously initiate a control interval for an appropriate rectifier by presetting a predetermined positive or negative digital number into the reversible counter associated with each phase. The reversible counter then counts down, if the preset number is positive, or up if the preset number is negative, during the control interval. During counting a digital speed error signal, derived from a previous comparison of a digital command with a digital feed-back signal indicative of motor speed is continuously compared with the contents of the reversible counter by the digital comparator. When the error exceeds the contents of the reversible counter, a firing pulse is generated and supplied to a positive or negative poled rectifier, firing the respectively poled rectifier in accordance with the positive or negative number.
Another prior art system of the digital type has been described in two papers, the first by R. D. Jackson and R. D. Weatherby entitled "DIRECT DIGITAL CONTROL OF THYRISTOR CONVERTERS" in "IFAC Symposium on Control and Power Electronics and Electrical Drives", October, 1974, held in Dusseldorf, Germany preprint Volume I, pages 431-441; and the second paper by F. Fallside and R. D. Jackson, entitled "DIRECT DIGITAL CONTROL OF THYRISTOR AMPLIFIERS" in the Proceedings of the Institution of Electrical Engineers-Control and Science, Volume 116, No. 5, pp. 873-878, May, 1969.
In the above mentioned papers the authors describe a study of a laboratory system of the direct digital control type to demonstrate the feasibility of direct digital control of controllable amplifiers such as silicon controlled rectifiers. In this system a programmed digital computer is employed to control the firing of rectifiers through an interface apparatus to control a resistive-capacitive load by the generation of firing pulses generated from the computer. The digital computer computes the firing angle which specifies a time to fire a particular rectifier. The system is synchronized to the zero crossings of the phase voltages of the AC source, such phase crossings initiating command signals for a sample and hold circuit of an analog to digital converter which measures the system load output voltage.
At the end of the analog to digital conversion a pulse is generated from the converter as an interrupt signal to the computer. This pulse starts the firing angle calculation. Subsequent to the initiation of the interrupt signal, the computer then reads the analog to digital converter at a specified time after the phase crossover instant. The computer then proceeds to calculate the firing angle or firing instant for the rectifier utilizing what the authors describe as a given firing law. In this calculation a control signal or value is developed which is continuously compared with a linear lookup table containing values defining the firing law until a match exists between the control signal and the contents of the table. When a comparison is reached, a firing pulse is generated by setting up the address of the rectifier to be fired and strobing a firing signal to the rectifier.
While the authors demonstrated the feasibility of direct digital control of controllable rectifiers, they also recognized that various practical difficulties were experienced in setting up a system of this type. This was apparent in the most obvious implementation in these experiments of the use of lookup tables which takes a considerable amount of computer time and severely restricts the amount of additional computation time which may be done by the computer in operating a real time system of this type.
Further, it is to be recognized that direct digital drive control systems must sample the load output voltage at a specified instant in time as related to a detected phase interval of the AC source, then perform the necessary firing angle calculations and fire a selected rectifier within that detected interval at a time early enough in the specified interval in order to accurately and fully control the retardation of the firing angle to obtain maximum transfer of power to the load.